Lots of people ask what the pressure of their chamber would be when they filled it from a 12 gram, so I thought I would make this chart.
The chart assumes that the CO<sub>2</sub> tank is at 800psi, which is its fill pressure at 73* Fahrenheit. There is an Excel document of this data at the end of this post.

4200/Chamber Volume In ci=Chamber Pressure -OR- 4200/C<sub>V</sub>=C<sub>P</sub>
For example:
You have a chamber volume of 41.3ci.
So it can be said that 4200/41.3=C<sub>P</sub>
4200/41.3 is equal to 101.6949152542373 For simplicity's sake we will round this to 101.7
This means that C<sub>P</sub>=101.7psi
You're 41.3ci chamber will have a pressure of 101.7psi when filled with one 12g CO<sub>2</sub> tank. Simple, eh?

When using CO<sub>2</sub> in a launcher, it is important to be aware of the fact that it expands and contracts rather rapidly acording to the temprature. For this reason, it is always a good idea to have a pressure gauge on any launcher that will be using CO<sub>2</sub>
The following graphs show both the Dynamic High and Dynamic Low of CO<sub>2</sub>. Remember, the higher the temperture, the more pressurize the gas
will be.

Finaly, if you dont know how to kind the volume of your chamber, use this formula:
Pi*RÃ‚Â²*L
And Example:
Your chamber is 36 inches long 3" PVC.
Your radius, then, is 1.5
Put into the Equation, it looks like this:
3.14*1.5Ã‚Â²*36
Remembering the exponents are figured first:
3.14*2.25*36
3.14*2.25=7.065, but we round to 7.
7*36=252
So, your chamber volume is equal to 252ci.
Now, obviously, this chamber is much too large to use for a spudgun powered by a single 12g, science your pressure wouldonly be about 17psi, but these were mearly numbers used as an example.
now, ideally, you would also take into account that volume that fittings add, but in most cases this isn't necessary.

The dynamic charts are based on what I was told (by experts) the highest and lowest average pressures are during very high/low temperatures. The high value would be about 105*F, and the low about 30*F. I was mearly stating with these charts, that you should more or less, have a chamber rated for the highest temp, and expect a lesspowerfull launch when it is cooler. I doubt many fire when it is over 105*, and when it is below 30*, a PVC launcher is no longer 'safe' to use, as it becomes brittle and fragile.

It is very true that CO<sub>2</sub> cools, the pressure will drop. That is why I stressed that these are not exact values, and that it is important to have a pressure guage on any CO<sub>2</sub> launcher. I doubt many people have a chamber that is <i>exactly</i> 40.00ci, and wait untill it is <i>exactly</i> 73*F outside to launch, let alone calculate the <i>slight</i> pressure drop when filling a chamber. If I was going to split hairs on this, there are lots of things I would exact before I took that aspect into acount. However, It is still a very important point for people to keep in mind.

Edit: I had to add subscript end tags, < /sub >, to your post because the text was too small to read. (plasticex)